But as the vault room and population getting larger and developed, maintaining vault become rather easier, so the need or impact of Lunchbox become smaller and smaller.
So most Lunchbox are purchase on early stage of vault. Player hardly buy more Lunchbox once vault pass some early development phase.
This is not good for both business and game itself in long run. There must be some need to spend money even at late stage of game.
Endgame objectives like collecting all the legendary dwellers or weapons are not enough. It is too fringe.
I suggest decoration to the rooms. The decoration will not have much practical value in game play. But see games like Farmville, where players pay huge amount of real money to decorate their virtual farm.
There should be more challenge and more way to continue spending money on this game at later stage of this game.